The present invention relates to a plate-type heat exchanger. Such heat exchanger is disclosed, for instance, in DE-B-1 064 539.
The invention may be mainly applied in the field of thermo-mechanical industry.
Plate-type heat exchanger devices are well known in the art.
These devices are constituted by a fixed and a movable endplate, one or both of them being provided, according to different applications, with inlet and outlet connections for fluids, and with a pack of metallic plates, generally stainless steel plates, disposed between said endplates.
Said metallic plates, which are bored close to their angled edges in order to allow the circulation of the fluids, constitute the surface of thermal exchange between the fluids and are provided with a series of ribs, in order to increase the surface and the fluid turbulence; said ribs are generally disposed according to a herringbone or a so-called laundress-board pattern.
Furthermore, the periphery of such plates is provided with sealing gaskets made of an elastic, rubber-like material, which delimit and physically separate the pair of channels formed inside of the heat exchanger and within which the fluids flow.
This type of device is generally used in applications of various kinds, e.g. for instantaneously producing sanitary hot water by means of a boiler, with or without buffers, or for swimming-pool heating by means of a boiler, or for a district-heating network.
Obviously, in accordance with particular applications, the devices are differently dimensioned in what concerns the surface and the number of plates, and are provided with different feeding circuits.
The peripheral sealing gaskets play a determinant role in the operativity of plate-type heat exchangers.
In fact, said gaskets need to be made of high quality materials allowing the gaskets to carry out a perfect seal at high temperatures and pressures, as well as to hold their efficiency even after thousands of hours of operation.
Furthermore, they should perfectly match to their suitable grooves which, on their turn, should allow the gaskets to be placed against possibly vertical walls, in order to assure a good support against the thrust which is exerted on the gasket from the inner side of the exchanger and this, particularly, in proximity of the fluid inlet bores.
The background art proposes different solutions in order to allow the gasket to be correctly placed inside of the groove.
A first known solution provides for the use of a suitable glue in order to stick the gasket on the groove's bottom.
This solution involves a plurality of disadvantages and drawbacks, which are due to high material and labour costs for carrying out the sticking operation, as well as to difficulties for upkeeping the plates in the case where a gasket has to be replaced.
In fact, a sticked gasket should be replaced by eliminating, by means of a suitable solvent, the old bonding agent; this operation should be carried out with care, in order to avoid damaging of the plate.
Thereafter, the groove should be carefully cleaned, and a new glue layer is placed in the same; then, a new gasket is placed into the groove and it has to be waited that it perfectly adheres to its seat.
This involves remarkable loss of time and high costs.
According to another known solution, which does not provide for the use of any bonding agents, the periphery of the gasket is provided with a series o substantially cylindrical protuberances which are suitable for being housed, by exerting a pressure on them, into corresponding holes which are present along the external periphery of the plate.
However, this solution involves drawbacks too, since it requires a very high precision for positioning the gasket relative to the plate, in such a way as each protuberance is aligned to a corresponding plate hole.
A further known solution provides for a series of tabs which are made of the same rubber material as the gasket and which are formed on the external periphery of the gasket.
These tabs are inserted, in operation, into cavities which are formed between the upper and lower surfaces of the plate, and they allow a sufficiently quick positioning of the gasket into the groove.
However, this solution involves drawbacks too, since the gasket is positioned in a rather rough way relative to the groove; furthermore, since the fastening tabs are necessarily made of rubber, they cannot show a high strength against the thermal and mechanical stress which a gasket normally undergoes during its working life.
As a result, the gasket gets frequently out of the groove, thereby causing leakages in the heat exchanger.
Another solution which has been proposed, and which exhibits the advantage of immediately positioning the gasket into the plate groove, consists in providing for a series of cavities which are disposed at given distances from each other in the gasket body, while the plate is provided with a series of projections which are obtained by automatically clamping and squeezing with tongs the groove walls, said projections being suitable for being inserted into said cavities and keeping then the gasket spot-fastened to the plate.
However this solution, which is extremely convenient for rapidly replacing a gasket, requires a height groove of at least 3 mm in order to reliably use the clamping and squeezing machine; now, in order to obtain high thermal exchange rates it is necessary that the height of the plate (and, more precisely, the height of the channels forming the thermal exchange circuit) does not exceed a given value, e.g. 2-2.2 mm. For this reason, this solution may be used, according to the background art, only for heat exchangers having a relatively low thermal exchange rate and a plate height exceeding 3 mm.
Another problem exhibited by plate-type heat exchangers is constituted by leakages in correspondence of the fluid inlet and outlet holes.
It has been attempted to solve this problem by using gaskets having a dovetail profile which expands when a fluid under pressure-is applied against the gasket itself.
However, also this solution has revealed to be unsatisfactory, since the metallic plate, which is not supported on its back by an element which may stand the pressure exerted by the fluid, bends in the region around the inlet and outlet holes; this may happen since the plates which are normally used are not sufficiently stiffened in proximity of the fluid inlet and outlet holes, and the dovetail-shaped gaskets cannot entirely compensate the plate deflection.
As a result, the heat exchanger shows leakages which are higher, the higher is the operating pressure; in no case, according to the background art, operating pressures exceeding 25 bar may be reached.
Thus, although the problem of positioning the gasket is well known in the art, nobody has proposed a reliable solution which allows a plate-type heat exchanger to be operated at very high operating pressures, e.g. 45-50 bar or even more.
Finally, a further problem raised by the plate-type heat exchangers known in the art is constituted by the difficulty of centering the various plates in respect of each other; it appears to be clear that, in the case where the plates are not perfectly centered in respect to each other, some regions are formed in which the channels of a plate do not perfectly match to the protuberances of an adjacent plate. This causes a faulty working of the exchange circuit and it is a source of potential fluid leakages from the heat exchanger.
WO-A-93/06 426 discloses a plate heat exchanger having a welded plate pair comprising first and second plates which are welded together at a contact region, a by-pass area being defined between the plates on the inboard side of the contact region. The plates are shaped and arranged so that a second by-pass area similar to the first is defined between the second -plate and the first plate of an adjacent, similar, plate pair.
EP-A-0 503 080 relates to a layer-built heat exchanger for exchanging heat between a first coolant and a second coolant. According to the invention, partitions are provided in a plate on a primary side and in a plate on a secondary side to form channels therein. A plurality of the plates on the primary side and of the plates on the secondary side are laminated one upon another with respective seal plates therebetween. Each of the partitions of the plates on the secondary side are located at positions opposed to each of the channels of the plates on the primary side and each of the partitions of the plates of the primary side are located at positions opposed to each of the channels of the plates on the secondary side.